Mercury’s Seasonal Na Exosphere Data from MESSENGER’s MASCS UVVS instrument Tim Cassidy, Aimee Merkel, Bill McClintock, Matt Burger Menelaos Sarantos, Rosemary Killen, Ron Vervack, Ann Sprague

From Potter et.al. (2001) Just how bright is the Na exosphere? We see the Na exosphere because Na scatters sunlight ~589 nm (yellow)

Messenger limb scan vs. Earth-based Na observations Messenger UVVS data is especially valuable because it gives high resolution vertical profiles (‘limb scans’) of the atmosphere Potter and Morgan, 1990 Killen et al., 2008

column density or radiance Altitude For Na, we will focus on near-surface (<1000 km) limb scans Of particular interest is the slope of the limb scan—which tells us the energy of ejected Na Gravity acts as an energy spectrometer. hot cold What is a limb scan and why is it useful? lines of sight

This talk is about the dayside, which is typically probed near the equator: Note: poles are harder to investigate with MESSENGER’s orbit. MESSENGER also has a lot of tail data, not presented here.

Chamberlain model: density ~ n 0 e -U/kT where U is the potential energy (times another factor called zeta…) Radiation acceleration term, analogous to U = -mgh Gravitational potential sunlight To get atmospheric properties we have fitted limb scans with a simple function, called a Chamberlain model. Chamberlain model fits give us two parameters: surface density and temperature Note: Radiation acceleration is up to ½ Mercury’s gravity

lines of sight Need to account for line of sight: line of sight column density = Integral of density over line of sight≈ surface density*2*K*H where K~Sqrt(pi*r/2H) H = kT/mg where g is the sum of gravitational and radial photon acceleration

I am going to focus on the dense lower exosphere in this talk What is it’s temperature? What is its density? How does it vary? And what do these tell us about the process that launches molecules off the the surface? Example limb scan fits:

Temperature is roughly constant Results, part I: Na temperature This excludes the high temperature ‘tail’ at high altitudes mentioned earlier. Modelers predicted a more variable temperature… Example: temperature at noon local time TAA 0° 180° Data from over 6 Mercury years

(some data points randomly excluded for clarity) And temperature is the same across dayside:

Killen et al., 1999: 1500 K (at equator) Compared with ground-based observations of the temperature: K 2008

We can compare with possible ejection mechanisms Thermal Desorption <700 K (and thermal accommodation) PSD Photon Stimulated Desorption similar to ESD, electron stimulated desorption Meteorite Impact Vaporization 1000s degrees Sputtering thousands to 10s of thousands of degrees Molecular dissociation (e.g. CaX  Ca + X + energy) 10s of thousands of degrees Experimental Data (Yakshinskiy and Madey, 1999 & 2004) 900 K Maxwellian PSD from ice Johnson et al., (2002) Conclusion: PSD is the best match to supply the near-surface exosphere temperature The temperatures we derive are similar to, but slightly colder, than Earth-based observations (Killen et al., 2008) There is no evidence of thermal desorption

But PSD would quickly deplete surface of Na, Na must be continually resupplied to surface by other processes such as impacts or ion-enhanced diffusion (e.g., Killen et al., 2008).

Results, part II: Na density

Plotting vs true anomaly angle shows pattern: (for this example, we use limbscans at 10:00 local time) Perihelion Aphelion TAA 0° 180° Data from over 6 Mercury years

Different local times have similar (but distinct) patterns (some data points randomly excluded for clarity)

Suggests correlation with radiation acceleration, as some ground based observations suggest TAA 0° 180° Potter et al., 2009

TAA 0° 180° Compared with ground-based data (Potter et al 2007) Lines show model of Smyth and Marconi (2005) It’s difficult to compare with ground based data, which tends to report disk-averaged quantities. A large effort would be required to do this.

TAA 0° 180° Exosphere content from ‘PSD enhanced’ modeling scenario Leblanc and Johnson, 2010 But perhaps the closest model:

Conclusions about Dayside Na These trends are consistent These limb scan column densities don’t change much from year to year: 20% standard deviation Strong evidence that PSD supplies lower dayside atmosphere -temperature (~1200K) -variation in noon density with TAA like Leblanc predicted in his ‘enhanced PSD’ simulation -no evidence of thermal accommodation/thermal desorption E.g., Noon, TAA ° There is no detailed comparison with models. Ground-based data does not seem to match our results. Observation geometry? Na has abundance comparable to O.

extra slides

Compared to atomic oxygen Estimate of O vertical column density: If O is hot (10's of thousands of degrees), then the vertical column density is of the same order as the line-of-sight density near the surface, which can found from the observed O emissions, about 4 Rayleighs, and the O g value (~1E-4/sec, Killen et al., 2009): =4 Rayleighs/g value*1E6 (/cm 2 ) = 4E10 cm -2 (regardless of the O temperature, this is an upper limit) vs Na:

Ca Ejected mostly from dawn and density peaks near perihelion—impact vaporization? High temperature (15,000-25,000 K): molecular dissociation of impact vapor Mg Uncertain mix of temperatures Nightside source needed Compared to Other Species Na Mg Ca Observed column densities: Na has a two components, two temperatures. It dominates near surface. Mg and Ca have single temperature. Mg dominates further from the surface.

Others suggested that same correlation Killen et al. (2008) Others did not:

--- Chamberlain with photon pressure 3000 K 1500 K 900 K Test: comparison of Chamberlain model with Matt’s Monte Carlo model, the gold standard Chamberlain model overestimates densities near dawn and dusk, where the Chamberlain model assumes no photon pressure effects (cos(chi)~0 in previous slide).

validating optical thickness correction